1. Field of the Invention
The present invention relates to an optical fiber for transmitting light, and a method of making the same.
2. Related Background Art
In the transmission of light using an optical fiber, the transmission loss (Rayleigh scattering loss) caused by Rayleigh scattering within the optical fiber becomes problematic. For this problem, an optical fiber which can reduce the Rayleigh scattering loss or a method of making the same has been proposed.
For example, xe2x80x9cSakaguchi, the Transactions of the Institute of Electronics, Information and Communication Engineers, 2000/1, Vol. J83-C, No. 1, pp. 30-36xe2x80x9d, discloses that the Rayleigh scattering loss in an optical fiber can be reduced by annealing the optical fiber after drawing. Namely, the Rayleigh scattering intensity within glass is not constantly fixed by materials thereof, but depends on a fictive temperature Tf which is a virtual temperature indicative of the randomness in the state of arrangement of atoms within glass. Specifically, the Rayleigh scattering intensity increases as the fictive temperature Tf within glass is higher (randomness is greater).
In this regard, when drawing an optical fiber preform upon heating, a heating furnace is installed downstream a drawing furnace and is heated such that the drawn optical fiber falls within a predetermined temperature range when passing through the heating furnace. As a consequence, the heating by use of the heating furnace prevents the drawn optical fiber from cooling drastically, whereby the optical fiber is annealed. Here, due to the structural relaxation of glass caused by rearrangement of atoms, the fictive temperature Tf within the optical fiber decreases, whereby the Rayleigh scattering intensity within the optical fiber is suppressed.
On the other hand, xe2x80x9cK. Tajima, NTT REVIEW, Vol. 10, No. 6, pp. 109-113 (1998)xe2x80x9d, discloses that the Rayleigh scattering intensity is similarly suppressed by drawing at a low temperature.
However, conventionally proposed manufacturing methods which are effective in lowering Rayleigh scattering loss, such as the above-mentioned manufacturing method having an annealing process using a heating furnace, and the like are not considered to fully realize the reduction of transmission loss in the optical fiber. In particular, there has been a problem that even optical fibers prepared by the same manufacturing method yield a case where the transmission loss reducing effect is obtained and a case where the transmission loss is hardly reduced or increases to the contrary, whereby the transmission loss reducing effect cannot be obtained reliably.
The inventor repeated diligent studies concerning causes for the unreliability of reduction in transmission loss mentioned above and the like; and, as a result, has found that, even when the same manufacturing method capable of reducing the Rayleigh scattering loss is used, the resulting effect of reducing the transmission loss greatly varies depending on the configuration of the optical fiber or optical fiber preform to which the manufacturing method is applied.
In view of the foregoing problems, it is an object of the present invention to provide an optical fiber which can reliably reduce the transmission loss caused by Rayleigh scattering loss and the like, and a method of making the same.
In order to achieve such an object, the first optical fiber in accordance with the present invention comprises a core region, and a cladding region disposed at an outer periphery of the core region, wherein the core average viscosity xcex70 at a cross section within the core region and the total average viscosity xcex7t at a total cross section combining the core region and cladding region together have a viscosity ratio Rxcex7=xcex70/xcex7t of 2.5 or less, and wherein the optical fiber has a Rayleigh scattering loss which is 95% or less of a predetermined reference value.
In the above-mentioned optical fiber, a manufacturing method effective in reducing the Rayleigh scattering loss such as the manufacturing method with annealing is used, or an optical fiber material adapted to lower the Rayleigh scattering loss is chosen, for example, whereby the Rayleigh scattering loss is reduced by at least 5% from a reference value indicative of the Rayleigh scattering loss in a conventional optical fiber, so as to become a value not higher than 95%. Further, the core region and cladding region of the optical fiber are configured such that the viscosity ratio Rxcex7 of the core to the total becomes 2.5 or less (Rxcex7xe2x89xa62.5).
When the viscosity ratio Rxcex7 is under such a condition, transmission loss components such as structural asymmetry loss other than the Rayleigh scattering loss can be reduced together with the Rayleigh scattering loss. Therefore, such a configuration of optical fiber can realize an optical fiber which can reliably reduce the transmission loss as a whole.
The above-mentioned optical fiber can be made by various manufacturing methods. As a specific manufacturing method thereof, a first method of making an optical fiber in accordance with the present invention comprises the steps of preparing an optical fiber preform comprising a core region and a cladding region provided at an outer periphery of the core region, in which the core average viscosity xcex70 at a cross section within the core region and the total average viscosity xcex7t at a total cross section combining the core region and cladding region together have a viscosity ratio Rxcex7=xcex70/xcex7t of 2.5 or less; and, when drawing the optical fiber preform upon heating, causing a heating furnace disposed downstream a drawing furnace to heat an optical fiber drawn by the drawing furnace such that the optical fiber attains a temperature within a predetermined temperature range.
When the optical fiber is thus annealed by use of the heating furnace disposed downstream the drawing furnace when drawing the optical fiber preform upon heating, the fictive temperature Tf within the optical fiber can be lowered as mentioned above, whereby the Rayleigh scattering loss can be reduced. Also, using an optical fiber preform satisfying the above-mentioned condition concerning the viscosity ratio R72 reduces the transmission loss component such as structural asymmetry loss occurring in the optical fiber upon drawing or annealing at the same time, thereby making it possible to yield a manufacturing method which reliably yields an effect of reducing transmission loss as a whole.
When a resin coating section for coating the drawn optical fiber with a resin exists in the above-mentioned method of making an optical fiber, the heating furnace disposed downstream the drawing furnace is preferably provided between the drawing furnace and the resin coating section.
In not only the manufacturing method based on annealing with the heating furnace but also other manufacturing methods effective in lowering the Rayleigh scattering loss, the effect of reducing the transmission loss as a whole can reliably be obtained in a similar manner when an optical fiber or optical fiber preform with a viscosity ratio Rxcex7xe2x89xa62.5 is used.
A second optical fiber in accordance with the present invention comprises a core region constituted by pure SiO2 or SiO2 doped with chlorine, and a cladding region disposed at an outer periphery of the core region, wherein the cladding region is doped with fluorine so as to yield an average relative refractive index difference xcex94nc satisfying the condition of
xcex94ncxe2x89xa7xe2x88x920.26%
when the relative refractive index difference in each part is defined as being expressed in terms of % with reference to the refractive index in pure SiO2; and wherein the optical fiber exhibits a Rayleigh scattering coefficient A of 0.81 dB/kmxc2x7xcexcm4 or less, or a transmission loss xcex11.00 of 0.82 dB/km or less at a wavelength of 1.00 xcexcm.
In an optical fiber (optical fiber preform) having a core made of pure SiO2 (pure silica) or a core similar thereto, the viscosity of the core region becomes higher than that in the cladding region doped with F or the like. Therefore, the stress generated within the optical fiber due to the tension at the time of drawing the optical fiber preform is concentrated into the core. Here, the dependence of transmission loss upon tension in thus obtained optical fiber becomes greater, thereby causing the transmission loss to increase.
In the above-mentioned optical fiber, the core region is made of pure SiO2, whereas the cladding region is configured such that the whole or part of the cladding region is doped with F (fluorine) by an amount within the range satisfying xcex94ncxe2x89xa7xe2x88x920.26%. By employing such a configuration, the optical fiber realizes the above-mentioned ranges of values, reduced from the reference values in the conventional optical fiber, concerning the Rayleigh scattering coefficient A or transmission loss xcex11.00, and transmission loss xcex11.55.
According to such a configuration of core and cladding regions, the upper limit of doping amount of F is given with respect to the cladding region, whereby the stress is dispersed into the cladding region. Therefore, the stress is restrained from being concentrated into the core in the optical fiber having the pure SiO2 core, whereby the dependence of transmission loss upon tension in thus obtained optical fiber can be reduced. As a consequence, transmission loss components such as structural asymmetry loss other than the Rayleigh scattering loss can be reduced together with the Rayleigh scattering loss. Therefore, such a configuration of optical fiber realizes an optical fiber which can reliably reduce the transmission loss as a whole.
The reference values for Rayleigh scattering coefficient A and transmission loss xcex11.00 are values of about 0.85 dB/kmxc2x7xcexcm4 and 0.86 dB/km, respectively, in an optical fiber having a pure SiO2 core (or a Cl-doped SiO2 core similar to the pure SiO2 core) obtained with a conventional configuration by a conventional manufacturing method. By contrast, the optical fiber having the configuration in accordance with the present invention attains the above-mentioned ranges of values concerning the Rayleigh scattering coefficient A and transmission loss xcex11.00, each reduced by at least about 5%.
A second method of making an optical fiber in accordance with the present invention comprises the steps of preparing an optical fiber preform comprising a core region constituted by pure SiO2 or SiO2 doped with chlorine, and a cladding region provided at an outer periphery of the core region, wherein the cladding region is doped with fluorine so as to yield an average relative refractive index difference xcex94nc satisfying the condition of
xcex94ncxe2x89xa7xe2x88x920.26%
when the relative refractive index difference in each part is defined as being expressed in terms of % with reference to the refractive index in pure SiO2; and, when drawing the optical fiber preform upon heating, causing a heating furnace disposed downstream a drawing furnace to heat an optical fiber drawn out of the drawing furnace such that the optical fiber attains a temperature within a predetermined temperature range, so as to yield an optical fiber exhibiting a Rayleigh scattering coefficient A of 0.81 dB/kmxc2x7xcexcm4 or less, or a transmission loss xcex11.00 of 0.82 dB/km or less at a wavelength of 1.00 xcexcm.
When the optical fiber is thus annealed by use of the heating furnace disposed downstream the drawing furnace when drawing the optical fiber preform upon heating, the fictive temperature Tf within the optical fiber can be lowered as mentioned above, whereby the Rayleigh scattering loss can be reduced. Also, using an optical fiber preform in which the core and cladding regions have the configurations mentioned above also reduces transmission loss components such as structural asymmetry loss occurring in the optical fiber upon drawing or annealing other than the Rayleigh scattering loss, thereby making it possible to yield a manufacturing method which reliably yields an effect of reducing transmission loss as a whole.
When a resin coating section for coating the drawn optical fiber with a resin is used in the above-mentioned method of making an optical fiber, the heating furnace disposed downstream the drawing furnace is preferably provided between the drawing furnace and the resin coating section.
In not only the manufacturing method based on annealing with the heating furnace but also other manufacturing methods effective in lowering the Rayleigh scattering loss, the effect of reducing the transmission loss as a whole can reliably be obtained in a similar manner when an optical fiber or optical fiber preform configured as mentioned above is employed.
Alternatively, the method of making an optical fiber in accordance with the present invention comprises the steps of preparing an optical fiber preform comprising a core region constituted by pure SiO2 or SiO2 doped with chlorine, and a cladding region provided at the outer periphery of the core region, wherein the cladding region is doped with fluorine so as to yield an average relative refractive index difference xcex94nc satisfying the condition of
xcex94ncxe2x89xa7xe2x88x920.26%
when the relative refractive index difference in each part is defined as being expressed in terms of % with reference to the refractive index in pure SiO2; and, when drawing the optical fiber preform upon heating, drawing the optical fiber preform at a tension within the range of 0.05 to 0.20 N, so as to yield an optical fiber exhibiting a Rayleigh scattering coefficient A of 0.81 dB/kmxc2x7xcexcm4 or less, or a transmission loss xcex11.00 of 0.82 dB/km or less at a wavelength of 1.00 xcexcm.
While the optical fiber preform (optical fiber) has a configuration restraining the stress from being concentrated into the core, tension control is effected such that the tension at the time of drawing is held within a preferable tension value range of 0.05 to 0.20 N, whereby an optical fiber having reduced the transmission loss can be obtained reliably.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.